In the production of oil and gas from a subterranean or subsea reservoir, the production flow from the well will almost always contain oil, gas, water, and a little sand. Therefore, terminal plants are commonly arranged in order to separate the different phases of the well production flow from each other. The separation is conducted in several steps by which a coarse separation is effected by means of gravity alone and where fine separation is facilitated by means of centrifugal forces and inertial forces in combination with gravity. The separation is conducted in large separators that may be arranged either horizontally or vertically. Examples of different embodiments of separators according to prior art technology are found in PCT/NO02/00379 and in U.S. Pat. No. 6,083,291.
Several steps of separation may take place in a separator. First, the gas enters the separator through an inlet conduit which in the case of vertical separators may be localized at the middle of the separator's vertical extension. At the inlet a baffle plate, a vane diffuser, or the like, is commonly arranged in order to distribute the fluid flow over the separator's cross-section. At this stage, the larger liquid drops are already separated out and fall into a liquid reservior in the lower part of the seperator.
The gas and remaining droplets continue upwards into what may be called a calm zone or settling zone in which additional droplets, due to gravity, fall into the liquid reservoir below, possibly subsequent to having been settled on the separator wall and drained down along the wall.
Close to the outlet conduit at the top of the separator, the gas is forced through droplet demisters of per se known technology in order to remove droplets that have not been removed by gravity.
It is very important that the inlet arrangement of the separator is appropriately designed, taking into consideration the separator's cross-sectional area, so that as much as possible of the liquid is separated out at this early stage, in order to avoid an excessive load of liquid into the droplet demisters. This consideration is particularly important for vertically arranged separators. Overload of demisters due to inadequately designed inlet arrangements and/or too small diameter for the separator compared to the gas flow rate is the main cause of problems encountered in a number of such installations. It is worth noting that in most vertically arranged separators the inlet arrangement relies upon gravitational forces alone to separate out liquid, which sets strict limitations to the maximum allowable flow rates of gas above which significant volumes of liquid will be drawn up to the demister equipment. Recently, different designs of cyclone inlet arrangements have been attempted in vertical separators, through some operational limitations still exist. The term “cyclone inlet arrangements”refers to designs by which the fluid charged to the separator is forced into a swirling motion so that a centrifugal force will act on the fluid in addition to gravity. PCT/NO02/00379 shows examples of different embodiments of cyclone inlet arrangements according to such prior art technology.
The most common inlet arrangement in gas scrubbers is denoted vane diffusers as also discussed in PCT/NO02/00379. Such a vane diffuser, known from Norwegian patent No. 164 960 and EP patent No. 195 464 is discussed in more detail with reference to FIG. 2 below. A vane diffuser is comprised of a vane arrangement arranged to reduce the fluid velocity in the inlet conduit before the fluid enters the separator. This is achieved by a plurality of parallel vanes that receive the inflowing fluid and divert the flow about 90 degrees to both sides. The parallel vanes together with a top plate and a bottom plate form channels which are curved and expand in the direction of the flow. Due to the fact that the curved vanes have even thickness in the direction of their length, it is difficult to make the fluid fill the entire flow channels. This is partly caused by a too abrupt expansion of the cross-section and partly caused by a pressure gradient across the flow direction that is a result of the curvature of the vanes. The highest pressure will be along the outer edge of each flow channel, also denoted the pressure side of the vane, while the lowest pressure is found along the inner edge of each flow channel, delimited by the adjacent vane's suction side. Therefore the theoretical available velocity reduction calculated from the cross-sectional area at the outlet of the diffuser is not achievable.
Due to inertial forces the majority of droplets will settle on the pressure side of the vanes and leave the vane's trailing edge in the form of a liquid film. Vane diffusers of prior art technology are not provided with any device to separate out any part of the entrained liquid from the gas before the fluids enter the separator.